1. Field of the Invention
The invention pertains to apparatus for detecting the passage of objects composed at least in part of ferrous or magnetic materials including iron and steel and more particularly relates to improved detection devices of the foregoing type exhibiting high immunity to spurious background or noise signals and minimum susceptibility to generation of false alarms or other undesired reactions.
2. Description of the Prior Art
Generally, prior art devices for detecting the presence of metallic ferrous or other objects have one or more disadvantages rendering them of little value in certain applications, especially in the reliable protection of expensive machinery from tramp metal, for instance. Prior art bridge devices often operate with excitation frequencies as high as 20 to 100 KHz and are therefore quite susceptible to the presence of moisture, vegetation, the operator, or the like. They impose on the operator the serious burden of continuous monitoring and adjustment of the calibration of the apparatus. Even at relatively lower excitation frequencies, such detectors are adversely affected by moist conditions, thereby rendering them unsuitable for use where high accuracy, reliability and freedom from generation of false alarms is demanded. Additionally, inductive loop detectors are highly susceptible to noise signals generated in moving parts of machinery, such as metal joints in conveyer belts and gear teeth or other similar projecting parts of rotating machine elements. Such machinery elements produce either false alarms in prior art detectors, or require unsatisfactory reduction of sensitivity. For example, little success has been shown in the prior art in the line of providing equipment for detecting tramp metal entering expensive machines such as farm tools because of the difficulty in separating the signals produced by the tramp metal from the high level of spurious electromagnetic noise produced by moving machine parts of the farm tool.